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Appendix

8is

Pretty, Wh. Sc., finds it possible to save much time and labour
when originating surface plates or straight-edges, by giving the
workman a tabulated statement of the order of procedure, so
arranged as to systematically reduce the errors of manipulation.
The plates having been stamped with numbers, (i), (2), (3), in
conspicuous places, and the planing-tool marks eliminated with a
smooth file, he follows this order:

(a) Using (i) as a standard : bed (2) to (i); bed (3) to (i);

then bed (2) to (3), working equally on each. 
(V) Using (2) as a standard: bed (i) to (2); (3) is already
bedded to (2); then bed (3) to (r), working equally on
each.

(f) Using (3) as a standard: bed (2) to (3); (i) is already
bedded to (3); then bed (r) to (2), working equally on
each.

(d) Using (i) as a standard : bed (3) to (i); (2) is already
bedded to (i); then bed (2) to (3), working equally on
each.

This cycle of operations is repeated until sufficient accuracy
is attained.   When straight-edges are being trued up they should
be  occasionally reversed  end   for end, so as to  eliminate all
possible errors.
V    P. 214. Screw-cutting.  There are no fewer than five